The present invention relates to electromagnetic imaging, more particularly to methods, systems, and computer programs for obtaining electromagnetic images of objects characterized by high-temperature, high-intensity light.
Various electromagnetic spectral band regions—for instance, visible light, radio, infrared, x-rays, etc.—have been used to detect objects or obtain images of objects. Active detecting/imaging devices (e.g., radar devices) transmit electromagnetic radiation (e.g., radio signals) to detect or image objects. Passive detecting/imaging devices do not transmit electromagnetic radiation, but instead receive naturally occurring electromagnetic signals that are emitted or reflected by objects.
In the electromagnetic spectrum, the infrared (IR) region is characterized by longer wavelengths than the visual region. The IR region extends between the visual region and approximately one millimeter in wavelength. The millimeter wave region has longer wavelengths than the IR region and shorter wavelengths than the radar (including microwaves and radio waves) region. Passive electro-optic devices and light-intensification devices operate in the visible spectrum. Passive IR devices are based on the phenomenon of natural radiation of IR energy by all “warm” objects in accordance with thermal radiative transfer physics.
The terms “electromagnetic image,” “photographic image,” and “photograph” are used synonymously herein to broadly refer to any image, still (e.g., “snapshot”) or moving (e.g., “video”), created by recording visible light or other electromagnetic radiation. For instance, electromagnetic radiation can be chemically recorded using a light-sensitive material such as photographic film, or electronically recorded using an image sensor.
Welding is a fabrication process typically involving the melting of metal workpieces. A filler material is added to form a “weld pool,” which cools to form a joint of the metal workpieces. Sometimes pressure is applied in conjunction with heat. A type of welding known as “arc welding” implements a welding power supply to create an electric arc (“welding arc”) between an electrode and a base material, thereby melting the metals at the welding point. It is often desirable to remotely observe in-situ, during fabrication, the dynamics of the welding arc and weld pool, in order to monitor the quality of the weld or to study phenomenological aspects of the weld.
Current methods of remotely observing optical welding systems focus primarily on using filtered visual light, with or without external lighting, to reduce or suppress the arc light interference. However, conventional approaches can suffer significant degradation of the overall image quality. Rejection by filters of specific wavelengths of light can be detrimental to complete imaging of the weld pool and welding arc. Camera systems that use external laser lighting to suppress specific arc radiation wavelengths have practical drawbacks such as high equipment costs, occupational safety issues associated with high power lasers, and system integration challenges; these drawbacks have largely relegated the use of external lighting welding arc imaging to research applications.
Attempts to image the welding process by mitigating radiation interference have employed such techniques as filtering, arc interruption, external lighting, and UDR photography. These techniques have worked with varying degrees of success. Examples of filtering are arc light band-pass filtering and neutral density filtering (i.e., signal attenuation). Arc light band-pass filtering typically involves rejection of a specific wavelength range of radiation. Neutral density filtering typically involves signal attenuation. Arc interruption typically involves current pulsing. External illumination has been provided, for instance, by back lighting using powerful lasers.
Charge coupled device (CCD) and complementary metal-oxide semiconductor (CMOS) video cameras operating in the visible to the near infrared spectra, equipped with band-pass and/or neutral density filters and external lighting, have been combined to image the weld pool and arc in real-time. Real-time image processing algorithms, such as ultra-dynamic range (UDR) photography, have been used to enhance quality of an image. UDR photography algorithms seek to optimize an image by piecing together the best portions of a series of consecutive photos taken over a range of exposure levels. However, UDR photography delays image broadcast and decreases video frame rate, resulting in “jumpy” video that ostensibly is real-time but actually is less than real-time.
A gas tungsten arc welding (GTAW) process has been observed using optical cameras, but this has required implementation of filtering technique(s) and/or image processing. Most commercially available visible-light to near-infrared arc cameras perform adequately with low-current welding processes that do not transfer metal across the arc, such as gas tungsten arc welding (GTAW). A gas metal arc welding (GMAW) process typically operates at a higher deposition rate and higher arc currents, thus creating a much hotter arc with higher radiance. Observation of a GMAW process using optical cameras thus requires an even greater degree of filtering and/or image processing.
Observation has been conducted of a GMAW-P process, which is a type of GMAW process that uses pulsed welding currents or synergic controlled pulsed currents. The rapidly changing pulsed welding currents create a fluctuating range of spectral radiation that band-pass filters cannot filter without eliminating some of the needed information to construct a complete image. UDR photography algorithms attempt to correct for this, but none are proficient at doing so; they tend to create choppy images due to decreased video frame rates.
Infrared (IR) weld monitoring has been used primarily for near-weld pool thermography, but not used specifically for weld pool imaging. Band-pass filtering of the arc light has been performed to improve the quality of the near-weld pool data; however, the radiance of the arc radiance, and the reflection of arc light off the surface of the weld pool, create noise that reduces quality of the image. Due in part to the difficulty of resolving the significant amount of IR interference caused by the intense, complex radiation of welding arcs, early researchers focused their attention on near-weld pool thermography rather than on weld pool or arc imaging. Moreover, the dramatic thermal gradients existing between welding arcs and surrounding material, coupled with IR detector limits of the time, further confounded both IR data and resulting imagery.
The following references, each of which is incorporated herein by reference, are informative regarding sensing and imaging of welds and welding: D. Farson, R. Richardson, and X. Li, “Infrared Measurement of Base Metal Temperature in Gas Tungsten Arc Welding,” Welding Journal, Welding Research Supplement, September 1998, Volume 9, pages 396s-401s; Lillquist, European Patent Specification EP 0092753 B1, “Infrared Sensor for Arc Welding,” publication date 11 Sep. 1988; X. Ma and Y. Zhang, “Gas Metal Arc Weld Pool Surface Imaging: Modeling and Processing,” Welding Journal, Supplement to the Welding Journal, May 2011, Volume 90, pages 85S-94S; Burke et al., U.S. Pat. No. 5,475,198, “Weld Pool Viewing System,” issue date 12 Dec. 1995.